The present invention relates generally to compositions having a nickel aluminide base and their processing to improve their properties. More specifically, it relates to tri-nickel aluminide base materials which may be processed into useful articles which have increased strength at room temperatures.
It is known that unmodified polycrystalline tri-nickel aluminide castings exhibit properties of extreme brittleness, low strength and poor ductility at room temperature.
The single crystal tri-nickel aluminide in certain orientations does display a favorable combination of properties at room temperature including significant ductility. However, the polycrystalline material which is conventionally formed by known processes does not display the desirable properties of the single crystal material and, although potentially useful as a high temperature structural material, has not found extensive use in this application because of the poor properties of the material at room temperature.
It is known that nickel aluminide has good physical properties at temperatures up to about 1100.degree. F. (600.degree. C.) and could be employed, for example, in jet engines as component parts at operating or higher temperatures. However, if the material does not have favorable properties at lower temperature, the aluminide may break when subjected to stress at such lower temperatures at which the part would be maintained prior to starting the engine or prior to operating the engine at the higher temperatures above 1000.degree. C. Any processing of such aluminides which significantly increases strength measured at room temperature while maintaining adequate ductility is valuable.
Alloys having a tri-nickel aluminide base are among the group of alloys known as heat-resisting alloys or superalloys. Some of these alloys are intended for very high temperature service where relatively high stresses such as tensile, thermal, vibratory and shock are encountered and where oxidation resistance is frequently required. Such alloys having good combinations of properties at temperatures up to about 1100.degree. F. are highly useful.
Accordingly, what has been sought in the field of superalloys is an alloy composition which displays favorable stress resistant properties not only at the elevated temperatures up to about 1100.degree. F. at which it may be used, as for example in a jet engine, but also a practical and desirable and useful set of properties at the lower temperatures to which the engine is subjected in storage and during warm-up operations.
Significant efforts have been made toward producing a tri-nickel aluminide and similar superalloys which may be useful over such a wide range of temperature and adapted to withstand the stress to which the articles made from the material may be subjected in normal operations over such a wide range of temperatures. The first set of problems of low strength and ductility at room temperature, that is in making such materials available, have been largely solved.
For example, U.S. Pat. No. 4,478,791, assigned to the same assignee as the subject application, teaches a method by which a significant measure of ductility can be imparted to a tri-nickel aluminide base metal at room temperature to overcome the brittleness of this material.
Also, copending applications of the same inventors as the subject application, Ser. Nos. 647,326 647,327; 647,328; 646,877 and 646,879 filed Sept. 4, 1984 teach methods by which the composition and methods of U.S. Pat. No. 4,478,791 may be further improved. These applications are incorporated herein by reference. These and similar inventions have essentially solved the basic problems of achieving high strength and ductility at lower temperatures such as room temperature. Also, there is extensive other literature dealing with tri-nickel aluminide base compositions.
For the unmodified binary intermetallic, there are many reports in the literature of a strong dependence of strength and hardness on compositional deviations from stoichiometry. E. M. Grala in "Mechanical Properties of Intermetallic Compounds", Ed. J. H. Westbrook, John Wiley, New York (1960) p. 358, found a significant improvement in the room temperature yield and tensile strength in going from the stoichiometric compound to an aluminum-rich alloy. Using hot hardness testing on a wider range of aluminum compositions, Guard and Westbrook found that at low homologous temperatures, the hardness reached a minimum near the stoichiometric composition, while at high homologous temperature the hardness peaked at the 3:1 Ni:Al ratio. TMS-AIME Trans. 215 (1959) 807. Compression tests conducted by Lopez and Hancock confirmed these trends and also showed that the effect is much stronger for Al-rich deviations than for Ni-rich deviations from stoichiometry. Phys. Stat. Sol. A2 (1970) 469. A review by Rawlings and Staton-Bevan concluded that in comparison with Ni-rich stoichiometric deviations, Al-rich deviations increase not only the ambient temperature flow stress to a greater extent, but also that the yield stress-temperature gradient is greater. J. Mat. Sci. 10 (1975) 505. Extensive studies by Aoki and Izumi report similar trends. Phys. Stat. Sol. A32 (1975) 657 and Phys. Stat. Sol. A38 (1976) 587. Similar studies by Noguchi, Oya and Suzuka also reported similar trends Met. Trans. 12A (1981) 1647.
More recently, an article by C. T. Liu, C. L. White, C. C. Koch and E. H. Lee appearing in the "Proceedings of the Electrochemical Society on High Temperature Materials", ed. Marvin Cubicciotti, Vol. 83-7, Electrochemical Society, Inc. (1983) p. 32, discloses that the boron induced ductilization of the same alloy system is successful only for aluminum lean Ni.sub.3 Al.
The subject application presents a further improvement in the nickel aluminide to which significant increased strength at lower temperatures has been imparted and particularly improvements in the strength of tri-nickel aluminide base compositions in the temperature range below about 600.degree. C.
It should be emphasized that materials which exhibit good strength and adequate ductility are very valuable and useful in applications below about 600.degree. C. 600.degree. C. is about 1137.6.degree. F. There are many applications for strong oxidation resistant alloys at temperature of 1100.degree. F. and below. The tri-nickel aluminide alloys which have appreciable ductility and good strength at room temperatures and which have oxidation resistance and good strength and ductility at temperatures up to about 1100.degree. F. are highly valuable for numerous structural applications in high temperature environments.